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European Association for the

Development of Renewable Energies, Environmentand Power Quality (EA4EPQ)

International Conference on Renewable Energies and Power Quality

(ICREPQ12)Santiago de Compostela (Spain), 28th to 30th March, 2012

Ferroresonant Configurations in Power Systems

V. Valverde, G. Buigues, A. J. Mazn, I. Zamora, I. Albizu

Department of Electrical EngineeringFaculty of Engineering of Bilbao, University of the Basque Country UPV/EHU

Alameda de Urquijo s/n, 48013 Bilbao (Spain)Phone/Fax number:+0034 946014200, e-mail: victor.valverde@ehu.es

Abstract. Over the last decades, power disturbances havebecome an important factor on the increase throughout electrical

networks, which can exert an important influence over powerquality ratios. Amongst them, it is worth mentioning

ferroresonance, which is a special case of resonance involvingnon-linear inductances that mainly affects the functionality of

transformers. Its effects are characterized by high sustainedovervoltages and overcurrents with maintained levels of currentand voltage waveform distortion, producing extremely dangerous

consequences. The first step against ferroresonance is always toprevent it from appearing, so it is important to identify thoseelectrical configurations prone to the appearance of the

phenomenon. Therefore, this paper analyzes differentferroresonant configurations that may take place in powersystems, highlighting some of the most relevant aspects of each

configuration.

Key words

Ferroresonance, Power Transformers, Voltage

Transformers, Overvoltage, Overcurrent.

1. IntroductionFerroresonance is a special case of disturbance that

involves high levels of overvoltage and overcurrentsdistortion. In general, the word ferroresonance includesall the oscillatory phenomena that take place in an electriccircuit comprising, at the least, a nonlinear inductance, acapacitance, a voltage source and low losses. Theappearance of the ferroresonant phenomenon provokesimportant oscillations that may lead to catastrophic failures

in the electrical power system. Furthermore, consideringthe large number of factors that may exert influence over

its appearance, most of them hardly quantifiable, itsconsequences have a higher degree of severity.Consequently, although it has been a widely analysedphenomenon over the last century, it is currently a problem

of the utmost importance in many electrical facilities.

In the last 25 years, the interest in this phenomenon hasundergone an important increase in the field of research in

electrical engineering. The constant evolution ofelectrical power systems has given rise to a significantincrease in the amount of failures caused byferroresonance. These failures have their origin in

different causes, amongst which should be included theincreased use of underground cables in primary circuits,single-phase operations, low-loss transformers, etc. [1].

The practical measures that can be taken in order tomitigate the ferroresonance are mainly based on specific

solutions adapted for each foreseeable situation.Although there are several techniques for damping the

ferroresonance oscillations [2], the first step againstferroresonance is always to prevent it from appearing, soit is important to identify those electrical configurationsprone to the appearance of the phenomenon.

This paper analyzes each of these ferroresonantconfigurations, highlighting their special characteristics.

2. Theoretical principles of ferroresonanceA. The ferroresonant circuit

The ferroresonance phenomenon is associated with thecoexistence, in the same electric circuit, of a non-linearinductance and a capacitor or capacitive load, connectedeither in series or parallel.

The effect of a nonlinear inductance is mainly found in

both power and voltage transformers. Moreover, thecapacitive effect necessary for the ferroresonancephenomenon can be provided by many elements. Someof the most common are: underground cables, overheadconductors, shunt capacitors, capacitive coupling ofdouble circuit lines, transformer stray capacitance and

grading capacitance of circuit breakers.

In addition, the presence of low-loss systems (i.e. low-resistive systems) increase the risk of ferroresonanceappearance. If system losses are considerable, the energy

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supplied by the source may not be sufficient to sustain theferroresonance. Some of the typical cases of low-losssystems are low-loss transformers design, no-load or low-load transformers (e.g. voltage transformers) or

transformers connected to low-loss circuits.

A combination of some of these elements and/or situationsmay provide the conditions for a ferroresonant circuit.Consequently, although this phenomenon is more frequentin distribution networks, due to the non-linear behaviour oftransformers and the capacitance of the undergroundcables, it may also appear in high voltage networks.

B. The ferroresonant phenomenon

This phenomenon is characterised by showing at least twostable steady-state responses for a particular range ofcircuit parameters: a ferroresonant one and a normaloperation one (non-ferroresonant). Furthermore,considering the dynamic characteristics of this nonlinear

disturbance, the abovementioned ferroresonant responsecan be classified into four groups, according to thefrequency content of the oscillation [3]:

Fundamental mode: periodic oscillations at thefundamental frequency of the power system.

Subharmonic mode: periodic oscillations at sub-multiple values of the fundamental frequency.

Quasi-periodic mode: non-periodic oscillations with adiscontinuous frequency spectrum.

Chaotic mode: non-periodic chaotic oscillations witha continuous frequency spectrum.

The jump from one stable steady-state response to anotherone is highly dependent on the initial conditions of thesystem. As a result, a little variation in the transient stateor in some of the parameters of the network may be the

trigger that causes a sudden jump between two possibleresponses, leading to the appearance of one of the fourferroresonant modes previously mentioned. This way, theevents that usually cause these kinds of variations can beclassified as being in one of two main groups:

Transient disturbances such as overvoltages,lightning, electrical faults or insulation failures.

Switching operations in different elements of thepower system, including transformers, capacitorbanks, loads, etc.

These transient events may give rise to a ferroresonantoscillation, depending on the situation of the power systemjust before the ferroresonance starts. Among the aspectsthat have influence over this situation, it is worthmentioning the following ones:

Power network characteristics: frequency and voltagelevels, shortcircuit power, system grounding, etc.

Properties of the voltage transformers magnetic core,especially those related to the saturationcharacteristic, hysteresis loop and eddy currentslosses.

Possible residual fluxes in the voltage transformersmagnetic core.

Circuits capacitance. Value of the voltage wave, in the instant of the

transient disturbance or when the switchingoperation takes place.

Due to the difficulty in controlling and quantifying eachand every one of these factors, ferroresonance is

frequently regarded as an unpredictable or randomphenomenon. Nevertheless, it can be characterised by

certain aspects that are usually dangerous for people, aswell as for the electrical equipment. Some of thesymptoms that may give an idea about the ongoingferroresonance are [4-5]:

Overvoltages and overcurrents with high levels ofharmonics.

Sustained levels of distortion. Loud noise (magnetostriction). Misoperation of protective devices. Overheating. Electrical equipment damage. Insulation breakdown. Flicker.

3. Ferroresonant Configurations in PowerSystems

There are several electric configurations that can lead toferroresonant circuits in power systems. In 2003 D.A.N.Jacobson [6] identified seven different classes or

categories of electrical systems prone to ferroresonanceappearance. This section analyses each of these seven

classes of ferroresonant systems. At the end, an eighthone has been included, regarding inductive grounded

systems.

A. Voltage transformer energized through the gradingcapacitance of open circuit breakers

This is a typical case of high voltage systems where a

circuit breaker is connected in series with an open circuit,having a phase-to-ground voltage transformer installed(Figure 1)

Fig. 1. Voltage transformer in series with an open circuit-breaker [3].

Opening the circuit breaker, phase-to-ground capacitanceis discharged through the voltage transformer that is

driven into saturation, provoking the ferroresonantoscillations. These oscillations are maintained by the

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energy supplied by the source through the circuit breakergrading capacitances [7-12].

The ferroresonant mode is generally fundamental or

subharmonic [8, 12], and its overvoltage can reach valueshigher than 4 p.u. (peak value).

B.

Voltage transformer connected to an ungroundedneutral system

When phase-to-ground voltage transformers are connectedto an ungrounded neutral system, a ferroresonant circuit isformed between the nonlinear inductance of the voltagetransformer and the phase-to-ground capacitance (Figure2) [4, 13-22].

Fig. 2. Ferroresonance in phase-to-ground voltage transformersof an ungrounded neutral system [21].

This is a typical situation in ungrounded distributionsystems with phase-to-ground fault detection protective

devices. These protection systems require the presence ofone phase-to-ground voltage transformer in each one of thethree phases. The transformer primary windings are wye-ground connected. The measure windings are ungroundedwye and the protective windings are open-delta connected,

so the delta voltage is used to relay operation or to detect aphase-to-ground fault [4, 15-16, 19].

Transient overvoltages or overcurrents due to switchingoperations or fault situations can initiate theferroresonance phenomenon in one of the transformers.

Values of ferroresonant overvoltages may exceed thenormal phase-to-phase voltage.

Under these situations, the protective devices associatedwith these voltage transformers can identify thedisturbance as an earth fault, causing undesiredmisoperations[15, 16].

The ferroresonant mode is usually fundamental,subharmonic or quasi-periodic.

C. Capacitor voltage transformerThe special design of capacitor voltage transformers(CVT) (Figure 3) makes them particularly prone to theferroresonance, so they often present a ferroresonantsuppression circuit. This circuit should not affect either the

transient response or the accuracy of the transformer, onlylimiting the duration of the ferroresonant oscillations [21,

23-27].

Fig. 3. Basic scheme of a capacitive voltage transformer.

Nowadays, there are mainly two types of ferroresonancesuppression circuits in CVTs: Active Ferroresonance

Suppression Circuits (AFSC) and Passive FerroresonanceSuppression Circuits (PFSC).

AFSC: Its design is based on a series-parallel RLCfilter.

PFSC: Its design is based on a saturable inductor inseries with a damping resistance.

AFSC are more effective in damping the ferroresonantoscillations than PFSC, although its influence on thetransient response of the transformer is higher. Bothsystems usually incorporate surge protection devices.

In recent years, CVT Technology is being used in powersubstation design instead of traditional powertransformers, in order to reduce the voltage level at somepoint of the line (e.g. rural power networks) [28]. Thistechnique involves considerable cost savings compared toconventional alternatives, but its main drawback is

related to being susceptible to ferroresonance [29-30].

D. Power transformer supplied accidentally on one ortwo phases

This type of electrical configuration is common in

grounded-wye distribution systems (more specifically inrural systems) that feed three-phase power transformersunder no-load or light-load conditions. Theferroresonance phenomenon appears after some type ofdisturbance or switching operation that provokes thepower transformer of the installation to be fed on one or

two phases (e.g. unipolar switchings, protection systems

based on fuses, broken phase conductor,) [13, 31-38]

If the primary winding of the power transformer isungrounded, the ferroresonant circuit is created throughthe phase-to ground capacitance(s) of the open phase(s)(Figures 4 and 5)

Fig. 4. Ferroresonant circuit in a transformer with delta primary

connection [3].

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Fig. 5. Ferroresonant circuit in a transformer with ungrounded-wye primary connection [3].

On the contrary, if the primary winding of the transformeris grounded, the ferroresonant circuit is created through thephase-to-phase capacitances (Figure 6).

Fig. 6. Ferroresonant circuit in a transformer with grounded-wye

primary connection [3].

If capacitances and primary windings have both the sameground connection (both ungrounded or both groundconnected) the ferroresonant circuit does not appear.

In general, the ferroresonant mode is fundamental,subharmonic or chaotic.

E. Power transformer supplied through a longtransmission line cable with low short-circuit power

Power transformers, under no-load or light-loadconditions, are prone to be driven into ferroresonance

when energized through a capacitive connection (longoverhead lines or underground lines) by a source with ashort-circuit power significantly lower than thetransformer rated power [39-42].

This type of configuration is common in medium voltage

networks (public, urban or industrial networks), afterservice restoration operations. It is also common in public

medium voltage networks of significant length or animportant ratio of underground cable.

Under these conditions, a parallel ferroresonant circuit isestablished between the lines capacitance and the

transformers magnetizing inductance. In these cases,ferroresonance is usually fundamental or quasi-periodic

[3].

F. Power transformer connected to a seriescompensated transmission line

Series compensation systems counteract the impedance of

transmission lines, reducing voltage variation andincreasing the power transfer capability. Furthermore, the

dynamic stability of the transmission grid is increased andthe load between parallel transmission lines is balanced.

Fig. 7 Representative circuit of a series compensation in a

distribution network [44].

The installation of capacitor banks in series-compensatedtransmission lines, along with the transformers of theselines, can induce the establishment of a ferroresonantcircuit (Figure 7). The ferroresonant mode is generallyfundamental or subharmonic [43-46].

G. Power transformer connected to a de-energizedtransmission line running in parallel with energized

line(s)

It is possible to find ferroresonance cases in power

transformers connected to de-energized transmissionlines of considerable length, which run parallel to anotherenergized line. Under these conditions, the capacitivecoupling between both lines can drive the powertransformer into ferroresonance [13, 47-51].

According to [50], the transmission lines must be high

voltage lines (above 115 kV), which run at least 10 or 20km in parallel.

The ferroresonant mode is generally fundamental orsubharmonic (order 3) [49], and it depends largely on thelength of transmission lines, being the subharmonic mode

more common for lengths over 150 km [48].

H. Inductive neutral networksFerroresonance appearance may occur when a neutralsystem is connected to ground through an inductiveimpedance. Consequently, the ferroresonant circuit isformed between the ground-connected reactor and thephase-to-ground capacitances of the network (Figure 8).

Fig. 8. Ferroresonant circuit in a MV network with Petersen coil[3].

In medium voltage networks, one of these neutral

systems (Petersen coil) is used to limit ground faultcurrents and get the fault self-extinguishing. The

ferroresonance appears after some type of disturbancethat can drive the coil into saturation.

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